This is an arbitrary limit by SqSp, among many. There should be more, and even more obvious there should be tags/categories in portfolios. As in: people should be able to filter and sort through portfolio items.
Also just heard back from the plugin developer. I asked her if the vertical nav can be customized. Here's what she said:
"The plugin can have the text left justified and you can change the background to be whichever color you choose. Most of the plugin uses CSS. However at times you may come across a situation where the header loads horizontally and then bounces to the vertical navigation. I have provided code to fix this issue, but you would need to have a Business Plan or Higher to use that portion of the code. "